Fast laser projectile detection system

ABSTRACT

The present invention is directed to an improvement in a system for measuring the ejection time and muzzle velocity of a gun. The measurement system utilizes a laser whose beam is split and then directed across the projectile ejection path to two photodetectors. When the gun is triggered, a projectile is fired and successively breaks the light paths between the laser beams and their associated photodetectors. The improvement comprises the use of analog and digital integrated circuit components in the detector portion of the measuring system. An analog device, a differential video amplifier, and a digital device, a differential dual line receiver, are combined with a photodetector and current amplifier to form the improved detector circuit.

Unite States atet [151 3,675,030 Tanenhaus July 4, 1972 54} FAST LASERPROJECTILE DETECTION 3,315,176 4/1967 Biard ..330 59 Y TEM 3,475,02910/1969 Hyman ..250 222 x [72] Inventor: Martin Tanenhaus, Suitland, Md.primary Examiner walter Stolwein [73] Assignee. The United states ofAmerica as Attorney-R. S. Sciascia and Thomas 0. Watson, Jr.

represented by the Secretary of the Navy [57] ABSTRACT [22] Filed: Jan.25, 1971 The present invention 15 directed to an improvement In a [2 l]App]. No.: 109,269 system for measuring the ejection time and muzzlevelocity of a gun. The measurement system utilizes a laser whose beam issplit and then directed across the projectile ejection path to [52] U.S.Cl. ..250/222 R, 250/209,332546//l278 two photodetectors. when the g istriggered a p j is [51] Int Cl G01 3/68 fired and successively breaksthe light paths between the laser 58] Fie'ld 206 beams and theirassociated photodetectors. The improvement 250/214 330/59, 356/28324/178 comprises the use of analog and digital integrated circuitcomponents in the detector portion of the measuring system. An

[56] References Cited analog device, a differential video amplifier, anda digital device, a differential dual line receiver, are combined with aUNITED STATES PATENTS photodetector and current amplifier to form theimproved detector circuit. 3,567,951 3/1971 Montgomery et al ..250/2223,430,106 2/1969 McDowell ..250/206 X 5 Claims, 2 Drawing Figures /9' n9/7 l0 l6 l3 STOP e COUNTER DETECTOR LASER -o.| START -o.|

FROM

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ATTORNEY FAST LASER PROJECTILE DETECTION SYSTEM STATEMENT OF GOVERNMENTINTEREST The invention described herein may be manufactured and used byor for the Government of the United States of America for governmentalpurposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION A. Field of the Invention The presentinvention relates generally to the field of ballistic measurements andmore particularly to an improved detection circuit in a system formeasuring the ejection time and muzzle velocity of a gun, especially arapid fire machine gun.

B. Description of the Prior Art In the prior art, there are varioussystems which are utilized for measuring the ejection time and muzzlevelocity of a gun. One of the most effective systems discovered so farfor measuring the above two characteristics of a gun is that disclosedin U.S. Pat. application, Ser. No. 815,689, filed Apr. 4, 1969, now USPat. No. 3,567,951. In the system'disclosed therein, a laser is utilizedwhose beam is split and then directed across the projectile projectionpath of two photodetectors. When the gun is triggered, a projectile isfired and a first counter is activated. When the projectile is ejectedfrom the gun barrel, it breaks the first light path between the laserand the first photodetector and turns off the first counter.Simultaneously, it initiates second counter which is deactivated whenthe light path to the second photodetector is broken by the projectile.The detector portion of that system utilizes essentially a photodetectorand a conventional capacitor to provide start and stop pulses tocounters from which the ejection time and muzzle velocity are obtained.

This method of activating the counters has been found to be occasionallyunreliable and inaccurate. The rise time of the pulse from the detectorused to activate the counters is not very high. Thus, the counters arenot precisely initiated resulting in imprecise time and velocitymeasurements. Further, in order to adjust the sensitivity of thephotodetector, the low load resistance is required to be greatlyincreased. This results, however, in a deterioration of the speed, thatis, the response time, of the photodetector and a correspondingdeterioration of the bandwidth of the photodetector signal. Thus, theoverall operation of the measurement system is adversely affected.Further, the reliability of the above described detection circuit isundesirably low due to the relatively small photodetectory signaloutput. Often, the signal output from the photodetector was insufficientto trigger the counters. In other cases, while the output signal fromthe photodetector was discemable, it was found to have dissipated tosuch a point in traversing the coaxial cable path to the utilizationequipment that it was insufficient to trigger the counters. Thisoccurred, for instance, during the measurement of the ejection time andmuzzle velocity of a machine gun located in a machine gun bay which wasphysically positioned quite distant from the utilization equipment,i.e., the counters and associated measuring equipment. Another problemwith the operation of the above device is the accidental triggering ofthe counters caused by unwanted pulses induced in the circuit by theambient noise present, for example, in a machine gun bay. Thus, theabove device has been found to produce inaccurate and unreliableresults.

SUMMARY OF THE INVENTION The instant invention has been found effectivein reducing or eliminating the above problems. The instant inventionsubstitutes integrated circuit components for the detector portion ofthe above measurement system. The integrated circuit components comprisean analog device, a differential video amplifier, a digital device, adifferential dual line receiver, and a current amplifier. Thedifferential video amplifier increases the amplitude of the outputsignal of the photodetector, thus insuring that me strength of thesignal will be sufiicient to trigger the counters. The sensitivity ofthe photodetector may be increased by the gain adjustment means of thedifi'erential video amplifier without significantly affecting theresponse time of the photodetector and without necessitating a change inthe load resistance. The difierential dual line receiver eliminates theproblem of accidental triggering of the counters due to erroneous pulsesinduced by ambient noise by its characteristic of high common moderejection. That is, ambient noise signals present on both input lines tothe differential dual line receiver are suppressed. The currentamplifier insures that the pulse detected will have sufficient strengthto trigger the counters after traversing the distance from the detectionpoint to the utilization point. The use of the integrated circuitcomponents improves the response time of the photodetector frommilliseconds to nanoseconds. The output pulse of the inventive detectorcircuit additionally has a rise time much greater than that of thesystem described above, thus, enabling more accurate triggering of thecounters.

OBJECTS OF THE INVENTION The overall object of the present invention isthe provision of improved means for measuring the ejection time andmuzzle velocity of a gun.

A specific object of the invention is the provision of means forincreasing the sensitivity of the detector portion of the measurementsystem without an adverse effect on the response time of the detector.

A further specific object of the invention is the provision of means foreliminating accidental triggering of the time counters of themeasurement system caused by erroneously induced pulses from ambientnoise.

Still another specific object of the invention is the provision of meansfor insuring accurate triggering of the time counters.

A still further specific object of the invention is the provision ofmeans to insure that the strength of the output pulse from the detectorportion of the measurement system will be sufficient to trigger the timecounters after traversing the distance from the detection point to theutilization point.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a preferredembodiment of a measurement system utilizing the inventive detector.

FIG. 2 illustrates a preferred embodiment of the inventive detectorcircuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1, which illustrates apreferred embodiment of a measurement system incorporating the inventivedetector, utilizes a laser 11 whose beam is directed over two parallelpaths 10 and 10' by beam splitter 12 and mirror 13. The beams 10 and 10'are directed across the path taken by the projectile 14 after beingfired from the gun 15. The laser beams are focused on the center of theprojectile path by lenses 16 and 16 and are then expanded by lenses l7and 17 and passed through optical filters 18 and 18, which pass onlythat light that has the frequency of the laser 11, to identical detectorcircuits l9 and 19. In order to obtain the most accurate reading ofejection time possible, the laser beam 10 is so positioned with respectto the gun 15 that the front of projectile 14 will break the laser beam10 as the back of projectile 14 is ejected from the gun 15 The two beampaths 10 and 10' are separated by a known distance. Upon interruption oftheir respective light beams, detectors l9 and 19' each emit a pulse totrigger or initiate counters 24 and 24'. In traversing the distance fromthe detection point, the location of detectors 19 and 19, to theutilization point, the location of counters 24 and 24, the output pulsesof detectors l9 and 19 travel through impedance matching resistors 22and 22 and across coaxial cables 23 and 23'.

FIG. 2 illustrates a preferred embodiment of the inventive detectorcircuit 19. The inventive detector circuit comprises three integratedcircuits 31, 33 and 34 and a photodiode or photodetector 30. While thelaser beam is illuminating the photodetector 30, the circuit is in astandby mode and no output signal from the detector will appear.However, upon interruption of the laser beam, the detector circuit willproduce an output pulse to initiate either counter 24 or 24.

The specific components of the inventive detector circuit may all beobtained as standard parts. For example, a PIN photodetector from UnitedDetector Technology may be employed as photodetector 30. Also aFairchild uA733C amplifier may be utilized as the difi'erential videoamplifier 31. Further, a National Semiconductor k) DM8870 receiver maybe utilized for the differential dual line receiver 33. Finally, aNational Semiconductor NHOOOZC amplifier may be utilized for the currentamplifier 34.

The operation of the measurement system is as follows. The gun iselectronically fired. The signal that fires the gun 15 also triggerscounter 24. When the back of projectile 14 is completely ejected fromthe gun, the front of the projectile interrupts laser beam 10. Thedetector immediately senses the interruption and emits an output pulseto stop counter 24 and to start counter 24'. The front of projectile l4subsequently will interrupt the laser beam 10. Detector 19, immediatelysensing this interruption, emits an output pulse to stop counter 24'.The reading from counter 24 provides the ejection time of the gun l5.Knowing the distance between paths 10 and 10 and the reading fromcounter 24, the muzzle velocity of the gun 15 can be calculated.

The operation of the inventive detector circuit is as follows. Thedifferential dual line'receiver 33 has two modes of operation, thestand-by-mode and the output mode. While the laser beam illuminates thephotodetector 30, the receiver 33 will be maintained in a stand-by modewith a 0.3 volt output. When the light illuminating the photodetector isless than that necessary to maintain the stand-by mode of thedifferential dual line receiver 33, as occurs when the laser beam isinterrupted by projectile 14, the receiver 33 switches to its outputmode, that is, to a 4.5 volt level.

The light intensity threshold setting of the photodetector 30 isdetermined by the gain adjust 32 of the differential video amplifier 31.Thus, the threshold setting or sensitivity of the photodetector 30 maybeadjusted by varying the gain adjust 32 of video amplifier 31.

The current amplifier 34 is used to protect the detector circuit fromcompacitive overloads and to sufficiently strengthen the output pulsefrom receiver 33 so that it will be able to trigger the counters 24 and24' after traversinG the cables 23 and 23'.

A characteristic of the integrated circuit component 33 is that it hashigh common mode rejection. Thus, the receiver 33 is effective tosuppress or eliminate ambient noise signals present on both input linesto the receiver and thereby prevent accidental triggering of thecounters 24 and 24'. The threshold level of the receiver 33 is preset.

The differential video amplifier 31 is capable of changing thesensitivity of the photodetector 30 without an appreciable correspondingchange in its speed. Previously in order to increase the sensitivity ofa photodetector in similar circuits, for example, the detector circuitof the previously mentioned patent application, the load resistance hadto be greatly increased, for instance, from 50 to 1,000 ohms. While thiswas effective in increasing the output signal, a correspondingdeterioration in the bandwidth and speed, i.e., response time, of thephotodetector followed. The differential video amplifier 31 eliminatesthis problem. The gain adjust 32 is capable of varying the sensitivityof photodetector 30 without an appreciable change in its speed orbandwidth. This results from the fact that in .varying the gain adjust32 from its minimum to its maximum positions, the bandwidth of the videoamplifier 31 will vary only from l20 megacycles to megacycles which willnot significantly affect the speed of the detector.

Thus, the inventive detector circuit rs capable of greatly improving thereliability and accuracy of the above described measurement system.Since it is obvious that many modifications, variations and other usesof the present invention are possible in light of the above teachings,it is to be understood that, within the scope of the appended claims,the invention may be practiced otherwise in as specifically described.

What is claimed is:

1. A device for measuring the ejection time and muzzle velocity of a guncomprising:

means for generating and directing first and second parallel beams ofcoherent light separated by a known distance across the projectile pathof a gun;

first and second photodiodes operative to receive said first and secondbeams of coherent light, respectively; first and second opticalfiltering means positioned adjacent to said first and secondphotodiodes, respectively, and operative to pass only that light whichhas the frequency of said generated coherent light;

first counting means electrically connected to said first photodiode,activated by the triggering means of said gun, deactivated by theinterruption of said first light beam by a projectile fired from saidgun and operative to indicate the ejection time of said gun;

second counting means electrically connected to said first and secondphotodiodes, activated by the interruption of said first light beam bysaid projectile, deactivated by the interruption of said second lightbeam by said projectile subsequently to the interruption of said firstlight beam by said projectile and operative to indicate the muzzlevelocity of said gun;

first and second differential video amplifiers each having a pair ofinput tenninals, one of each of said pair of terminals being connectedto said first and second photodiodes respectively;

first and second sources of DC voltage connected to the other inputterminals of said pair of terminals, and to said first and secondphotodiodes respectively; and,

first and second means electrically connected to said first and seconddifferential video amplifiers, for providing digital output signalsindicative of the presence or interruption of said light beams on saidphotodiodes and for suppressing electrical signals resulting from thepresence of ambient noise.

2. A device as recited in claim 1 further comprising first and secondcurrent amplifying means electrically connected to said first and seconddigital output means, respectively.

3. A device as recited in claim 2 in which said first and seconddifferential video amplifiers, first and second digital output means andfirst and second current amplifying means are integrated circuitcomponents.

4. A device as recited in claim 3 further comprising first and secondmeans for varying the amplification characteristics of said first andsecond differential video amplifiers, respectively, over a range ofvalues.

5. A device as recited in claim 1 in which said means for generating anddirecting first and second parallel beams of coherent light comprise:

a laser;

beam splitting means optically connected to said laser and operative topass a first beam of coherent light and to reflect a second beam ofcoherent light;

reflective means optically connected to said beam splitting means andoperative to direct said second beam of coherent light along a pathparallel to the path of said first beam of coherent light; and first andsecond means in the paths of said first and second beams of coherentlight, respectively, for focusing said light beams across the projectilepath of said gun.

1. A device for measuring the ejection time and muzzle velocity of a guncomprising: means for generating and directing first and second parallelbeams of coherent light separated by a known distance across theprojectile path of a gun; first and second photodiodes operative toreceive said first and second beams of coherent light, respectively;first and second optical filtering means positioned adjacent to saidfirst and second photodiodes, respectively, and operative to pass onlythat light which has the frequency of said generated coherent light;first counting means electrically connected to said first photodiode,activated by the triggering means of said gun, deactivated by theinterruption of said first light beam by a projectile fired from saidgun and operative to indicate the ejection time of said gun; secondcounting means electrically connected to said first and secondphotodiodes, activated by the interruption of said first light beam bysaid projectile, deactivated by the interruption of said second lightbeam by said projectile subsequently to the interruption of said firstlight beam by said projectile and operative to indicate the muzzlevelocity of said gun; first and second differential video amplifierseach having a pair of input terminals, one of each of said pair ofterminals being connected to said first and second photodiodesrespectively; first and second sources of DC voltage connected to theother input terminals of said pair of terminals, and to said first andsecond photodiodes respectively; and, first and second meanselectrically connected to said first and second differential videoamplifiers, fOr providing digital output signals indicative of thepresence or interruption of said light beams on said photodiodes and forsuppressing electrical signals resulting from the presence of ambientnoise.
 2. A device as recited in claim 1 further comprising first andsecond current amplifying means electrically connected to said first andsecond digital output means, respectively.
 3. A device as recited inclaim 2 in which said first and second differential video amplifiers,first and second digital output means and first and second currentamplifying means are integrated circuit components.
 4. A device asrecited in claim 3 further comprising first and second means for varyingthe amplification characteristics of said first and second differentialvideo amplifiers, respectively, over a range of values.
 5. A device asrecited in claim 1 in which said means for generating and directingfirst and second parallel beams of coherent light comprise: a laser;beam splitting means optically connected to said laser and operative topass a first beam of coherent light and to reflect a second beam ofcoherent light; reflective means optically connected to said beamsplitting means and operative to direct said second beam of coherentlight along a path parallel to the path of said first beam of coherentlight; and first and second means in the paths of said first and secondbeams of coherent light, respectively, for focusing said light beamsacross the projectile path of said gun.